The role of canine reduction in diagnosing the earliest hominins: lessons from a Miocene ape

Ardipithecus ramidus was diagnosed as a hominin largely based on changes to its C/P3 honing complex. These consisted of maxillary canine crown height reduction, an alteration in the morphology of the maxillary canine and mandibular P3, a decrease in sexual dimorphism of the maxillary canine, and a loss of honing. This diagnosis has been questioned given the apparent parallel evolution of canine crown height reduction in several late Miocene Eurasian hominoid taxa. The aim of this study was to evaluate whether the reduced maxillary canines possessed by Oreopithecusbambolii undermine the assertion that canine reduction in Ar.ramidus is phylogenetically informative.

We used phylogenetic comparative methods to determine whether or not O. bambolii possessed maxillary canines that are shorter than expected for a catarrhine of its estimated body mass. Canine reduction was also assessed in Ar. ramidus, Australopithecus afarensis, and Au. anamensis.

The canines of all three hominins were found to be shorter than expected for estimated body mass. The maxillary canines of O. bambolii are not shorter than expected for a catarrhine of its estimated body mass. This taxon retained substantial canine sexual size dimorphism, lacked the morphological changes to the maxillary canine observed in hominins, and retained a honing mechanism. Our results imply that the canine reduction present in O. bambolii was not homologous to that of the hominins. Based on these findings we suggest that canine size reduction is a character that may continue to be used to attribute taxa to the hominin clade.

Research supported by an Institute of Human Origins Graduate Fellowship to HG.